The enormous demands placed on the world's fossil fuel reserves have led to concerns regarding global warming, energy security and environmental pollution. Researchers continue to seek alternative fuel sources. Molecular hydrogen is ideal in this regard because it is lightweight, abundant, has more than three times the energy density by mass than currently used hydrocarbon fuels such as gasoline, and its only combustion product (water) is environmentally benign. Despite the advances made in fuel cell technology and hydrogen production, storage remains a great hurdle. See, e.g., R. H. Wiswall et al., Science, 186, 1158, 1974; S. Orimo et al., Chem. Rev., 107, 4111, 2007, and L. K. Heung, On-board Hydrogen Storage System Using Metal Hydride, HYPOTHESIS II, 1, 1997. Using current technology, hydrogen storage has a low energy storage density by volume relative to hydrocarbon fuels. Therefore, with all other factors being equal, in order to store the same amount of energy, hydrogen storage requires a much larger and heavier storage tank than hydrocarbon fuel storage.
Gravimetric capacity is a measure of the amount of hydrogen that can be stored per unit mass of the storage system. Volumetric capacity is a measure of the amount hydrogen that can be stored per unit volume of the storage system. The United States Department of Energy (DOE) has set targets for hydrogen storage. The 2017 target set by the DOE for hydrogen storage is 5.5 wt. % and 40 kg/m3 volumetric adsorption for a fully reversible system operating near room temperature. The ultimate goals are 7.5 wt % and 70 kg/m3.
To date no technology has satisfied all the requirements set out by the DOE. Some technologies being considered involve the use of chemical carriers such as alloys, adsorbents such as amorphous carbons (see, e.g., R. Yang et al., J. Am. Chem. Soc., 131, 4224, 2009), zeolites (see, e.g., A. Pacula, et al., J. Phys. Chem. C, 112, 2764, 2008) and metal organic frameworks (MOFs) (see, e.g., K. M. Thomas, Dalton Trans., 1487, 2009; S. S. Kaye et al., J. Am. Chem. Soc., 129, 14176, 2007, and N. L. Rosi et al., Science, 300, 1127, 2003).
The use of metal hydrides, such LiH and NaAlH4 is thwarted by heat management issues and problems with slow kinetics and/or reversibility. For example, when hydrogen reacts with magnesium or a sodium-aluminum alloy to give a metal hydride such as MgH2 and NaAlH4, significant amounts of heat are given off. When this heat is produced, a cooling step must be carried out to prevent a significant rise in temperature in the system, and this cooling step constitutes an energy loss to the system. Furthermore, heating is typically necessary to remove the hydrogen when required. This is an artifact of the high enthalpies of hydrogen binding (>60 kJ/mol) typical of hydrides such as MgH2 and NaAlH4.
Compression techniques have been used to increase gas pressure and improve the energy storage density by volume for hydrogen. This allows for the storage tanks to be smaller. However, compressing hydrogen requires a significant amount of energy, often accounting for as much as 30% of the stored energy. Furthermore, large pressure vessels are required for such compression techniques.
Another technique for storing hydrogen involves converting hydrogen gas to liquid hydrogen. This technique requires cryogenic storage because hydrogen has a very low boiling point (−252.88° C.). The liquefaction of hydrogen requires a large amount of energy to maintain these extremely low temperatures. Furthermore, the storage tank for liquid hydrogen requires complex and expensive insulation in order to prevent the liquid hydrogen from evaporating. In addition, liquid hydrogen has a lower energy density by volume than hydrocarbon fuels, such as gasoline, by a factor of about 4.
Physisorption materials, such as amorphous carbons and metal organic frameworks (MOFs), achieve promising storage capacities at temperatures of 77 K, but typically lose approximately 90% of their performance at room temperature due to low heats of adsorption (typically 5-13 kJ/mol H2). See, e.g., A. Dailly et al., J. Phys. Chem. B, 110, 1099, 2006, J. Rowsell et al., Angew. Chem., Int. Ed., 2005, 4670, 2005. In order to achieve the DOE target under ambient conditions, the ideal H2 binding energy is predicted to be in the range of 20-30 kJ/mol per hydrogen molecule See, e.g., R. Lochan et al., Phys. Chem. Chem. Phys., 8, 1357, 2006.
Moreover, energy production costs for the preparation of hydrogen storage materials may be an important factor. There is, therefore, a need for improved, lower cost materials that can be used as hydrogen storage systems.